1. Field of the Invention
The invention relates in general to a modified hyper-branched polymer and proton exchange membrane applied with the same and method for manufacturing the proton exchange membrane, and more particularly to the modified hyper-branched polymer which has improved dimensional stability and high proton transporting rate at low humidity, and method for manufacturing the same.
2. Description of the Related Art
Nafion® (sulfonated tetrafluorethylene copolymer) is a conductive polymer developed by Dupont in the 1960's. This compound is also called ionomer due to its ionic property. Nafion has the unique quality of being microphase separated into hydrophobic and hydrophilic domains composed of backbone rich and sulfonic acid side chain rich regions respectively. The ionic property of Nafion comes from the sulfonic acid moiety (side chains) grafted on the terminals of the polytetrafluorethylene (backbone). Because of great thermal stability and mechanical durability, Nafion has attracted extensive interest and became one of important materials studied by the researchers related to the technology of the Proton Exchange Membrane Fuel Cells (PEMFC).
Protons from the sulfonic acid moiety of Nafion are able to transfer from one sulfonic acid to another for proton exchange. However, electrons and anions (negatively charged ions) of Nafion cannot be transferred between the sulfonic acids as protons.
Although Nafion has many advantages, it shows considerable dimensional changes after heat treatment. U.S. Pat. No. 4,983,690 to Cameron et al. discloses a conductive polymer blend of Nafion and bismaleimide, and method of producing the same, for improving the mechanical properties of the produced Nafion-based membrane. Unfortunately, however, it had been proved by the experiments that the membrane made by the polymer blend disclosed by Cameron couldn't solve the problem of dimensional changes effectively. Moreover, proton transfer/transport is also an important factor to be considered when Nafion-based membrane is adopted for being the proton exchange membrane of the fuel cell. It had been proved that the proton transporting rate in the polymer blend of Nafion and bismaleimide is lower than that in Nafion.
Accordingly, it would be desirable to develop a novel material with excellent properties of great dimensional stability and high proton transporting rate, particularly high proton transporting rate at low humidity.